Personal Programming Notes

To err is human; to debug, divine.

Docker: Files Not Found in Docker Container

In this post, we look into a perplexing issue that happens often when we try to extend an offical image. In summary, there is currently no way to add additional content into VOLUME directory in a trivial way. If you unknowingly adds files into a folder that has been used as a VOLUME mount point in a Docker image or one of its base images, the files cannot be found for seemingly no reason.

Problem description

Let’s say we created the following Dockerfile for container, extending on top of a base Docker image:

Dockerfile of the extended image
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FROM docker.registry.company.net/base
MAINTAINER myemail@company.net

RUN ssh-keyscan -H github.company.net >> /home/jenkins/.ssh/known_hosts

RUN mkdir -p /home/jenkins/.m2 /home/jenkins/store
COPY settings.xml /home/jenkins/.m2/settings.xml

RUN openssl s_client -connect nexus.company.net:443 < /dev/null | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > /home/jenkins/public.crt \
    && /opt/jdk-latest/jre/bin/keytool -import -noprompt -storepass change_this -alias nexus.company.net -keystore /home/jenkins/cacerts -file /home/jenkins/public.crt

When we try to build the above image with docker build, we should expect several files dropped into the home /home/jenkins folder. In fact, we can add several echo commands into the above Dockerfile to verify in the build log that the files are actually created. However, when we start running a container from this new Docker image using docker run, the files simply don’t exist in /home/jenkins.

This was very perplexing at first. Struggling with different options of rebuilding (docker build) and re-running (docker run) gives no different outcomes. It eventually turns out that /home/jenkins is mounted as a volume in the base image or one of its base images. If you have access to the base images' Dockerfiles, you should expect to find the following lines in one of the base Dockerfiles:

Base Dockerfile
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...
VOLUME /home/jenkins
WORKDIR /home/jenkins
...

Otherwise, you can verify with the following docker inspect command when the Docker container is still running:

Show Volumes
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mymac:docker tdongsi$ docker inspect --format { {.Config.Volumes} } 683bb8ce246a
map[/home/jenkins:{}]

This problem is already seen and reported in this issue. There have been suggestions that VOLUME directive in Dockerfile is a mistake. It should be an option/directive when running docker run, not when building images.

Resolving problem

The above problem can be resolved by simply adding files into another path that has NOT been used as VOLUME. If the specific VOLUMEed path (/home/path in the example) must be used, you can also use docker copy to add files into a running container (see last post).

References